1. Field of the Invention
This invention relates to a comb filter, and particularly to a comb filter for deriving a carrier chrominance signal reproduced from a video tape with crosstalk components from adjacent tracks being suppressed according to the amount of line correlation error in a video signal reproducing apparatus such as a video tape recorder or the like.
2. Description of the Prior Art
Various types of comb filters are known which are used for suppressing crosstalk components included in a video signal wherein the PS (phase shift) method is used. The PS method has been described by the assignee of the present invention, in Japanese patent application provisional publication No. 56-9073 (for an NTSC signal) and in Japanese Patent Publication No. 55-32273 (for a PAL signal). FIG. 3 is a block circuit diagram of an example of a prior art comb filter used for suppressing crosstalk components included in a video signal processed in a playback system such as an NTSC system video tape recorder with color blurring on the resultant displayed image being eliminated by selecting either filtered or non-filtered playback carrier chrominance signal according to the degree of error in line correlation between the carrier chrominance signal in successive one horizontal scanning intervals, referred to in the following as the line correlation error.
In a circuit of FIG. 3, a playback luminance signal applied to an input terminal 1 is delayed by a delay circuit 2 having a delay time equal to one horizontal scanning interval (hereinafter referred to as 1 H). The delayed signal and the playback luminance signal are fed to a subtractor 3. The subtractor 3 subtracts the delayed luminance signal from the playback luminance signal. As a result, a signal of line correlation error is detected at an output of the subtractor 3. This signal will be referred to as a line correlation detection error signal. This line correlation detection error signal is fed to a full-wave rectifier 4 which full-wave rectifies the line correlation detection error signal. The rectified signal is formed into a pulse signal for controlling switch circuit 6 by a waveform shaping circuit 5 which includes a level comparator comparing the rectified signal with a predetermined value. The switch circuit 6 transfers only a signal coming to a terminal t1 according to the pulse signal when the amount of the line correlation detection error signal is larger than the predetermined value and transfers only a signal coming to a terminal t2 when the amount of the line correlation detection error signal is smaller than the predetermined value. On the other hand, a playback carrier chrominance signal is applied to the input terminal 7, a delay circuit 8, and a subtractor 9. The delay circuit 8 delays the playback carrier chrominance signal by 1 H and sends a delayed signal to the subtractor 9. The subtractor 9 subtracts the delayed signal from the non-delayed chrominance signal and outputs a signal to the terminal t2. The delay circuit 8 and the subtracter 9 make up a comb filter.
Such a comb filter has a frequency/attenuation characteristic whereby if there is a sufficient degree of line correlation between the carrier chrominance signal in successive 1 H intervals, each component of the carrier chrominance signal having a frequency that is an odd-numbered multiple of fH/2 (where fH is the horizontal scanning frequency) is enhanced. The playback carrier chrominance signal, which has been phase-shifted by 180.degree. between each pair of successive 1 H intervals, is thus enhanced, while the crosstalk components from adjacent tracks which are in-phase in successive 1 H intervals will mutually cancel. A playback carrier chrominance signal with crosstalk components suppressed and a high signal/noise ratio is thereby obtained. Therefore, when the amount of line correlation detection error signal is smaller than the predetermined value, the playback carrier chrominance signal is processed through the comb filter for removing crosstalk components and the subtractor 9 sends the processed carrier chrominance signal to an output terminal 10 through the terminal t2. On the other hand, when the amount of the line correlation detection error signal is larger than the predetermined value the playback carrier chrominance signal is sent to the terminal 10 through the terminal t1 without passing through the above-mentioned comb filter.
The reason for providing the switch circuit 6 is as follows:
There is a problem that the playback carrier chrominance signal processed through the comb filter reproduces a displayed image with color blurring when the degree of line correlation is low. Such problem affects more heavily the image quality compared with the crosstalk problem when the degree of line correlation is low. PA0 Therefore, when the degree of line correlation is low the playback carrier chrominance signal is by-passed the comb filter and not processed through the comb filter. PA0 However, with the prior art comb filter described above, since the luminance signal may vary slightly around the predetermined value, unnecessary switching occurs. Such switching causes switching noise. Therefore, in the prior art comb filter, there is a drawback that the switching noise is conspicuous on the displayed image.